Humidity indicators

ABSTRACT

Silica-based carrier impregnated with iron(III) or iron(II) salts functions as a humidity indicator giving a yellow/amber to nearly colourless colour change as the carrier humidifies.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. Ser. No. 09/959,121,filed Oct. 22, 2001, which is the National Phase of PCT/GB00/01390,filed Apr. 12, 2000 designating the United States, and which waspublished in English. These applications, in their entirety, areincorporated herein by reference.

[0002] This invention relates to silica-based humidity indicators.

[0003] Cobalt chloride indicator gels are used in a range ofapplications, e.g. to indicate moisture change in gas drying columns.Other drying applications include their use in transformer breathers,tank breathers, in the protection of electronics and telecommunicationsystems and in laboratory desiccators. It is estimated thatapproximately 2000 tonnes of cobalt chloride indicator gel are usedannually on a global basis.

[0004] Cobalt-containing gels for use as humidity indicators have beendisclosed in U.S. Pat. No. 2,460,071 (disclosing cobalt chloride), U.S.Pat. No. 2,460,069 (disclosing cobalt bromide), U.S. Pat. No. 2,460,073(disclosing cobalt iodide), U.S. Pat. No. 2,460,074 (disclosing cobaltthiocyanate), U.S. Pat. No. 2,460,065 (disclosing cobalt sulphate) andU.S. Pat. No. 2,460,070 (disclosing cobalt phosphate).

[0005] Indicator silica gel is currently produced by impregnatinghumidified silica gel or a silica hydrogel with a cobalt chloridesolution to produce a dry granular end-product which contains a minimumof 0.5% cobalt chloride and which is blue in colour, changing to pinkwhen water has been adsorbed. Humidified gel is silica gel that has beensaturated with water from the vapour phase in order to avoiddecrepitation upon impregnation. If the cobalt chloride solution isadded directly to the dried gel, the grain size is reduced.

[0006] Cobalt chloride has recently been classified as a Category 2carcinogen (notification from the EEC, 15/12/98) with the consequencethat the use of cobalt chloride indicator gel in industrial applicationswill require much tighter control to ensure exposure limits are strictlycontrolled. If acceptable alternatives to the cobalt chloride indicatorgel were not available to indicate when saturation had occurred ingas/air drying applications, for instance, this could have seriousimplications on the users' downstream processes, e.g. corrosion throughmoisture damage.

[0007] U.S. Pat. No. 2,460,072 and U.S. Pat. No. 2,460,067 also disclosecopper(I) chloride and copper(II) bromide, respectively, but thesecompounds are not considered suitable candidates for a commercial silicagel-based humidity indicator because of potential toxicity andenvironmental considerations.

[0008] It has been demonstrated that the vanadium compound VOCl₃, whenimpregnated into silica gel gives a colour change from colourless toyellow to orange to red to brown as humidity increases—see the followingreferences:

[0009] Belotserkovskaya et al., “Indicator properties ofvanadium-modified silicas and zeolites” Zh. Prikl. Khim. (Leningrad),63(8), 1674-9;

[0010] Malygin, A. A. “Synthesis and study of physicochemical propertiesof vanadium-containing silica—a humidity indicator”, Sb. Nauch. Tr. VNIILyuminoforov I Osobo Chist. Veshchestv, 23, 24-8; and

[0011] Malygin, A. A. et al, “Study of properties of vanadium-containingsilica gel”, Zh. Prikl. Khim. (Leningrad), 52(9), 2094-6.

[0012] However, VOCl₃ is corrosive, toxic and difficult to prepare andhandle.

[0013] The present invention therefore addresses the problem ofproducing an alternative, safe indicator gel to those which arecobalt-based or contain a transition metal salt which is considered tobe toxic.

[0014] According to one aspect of the present invention there isprovided a humidity indicator compound comprising a silica-based carriercontaining an iron(II) and/or iron(III) salt or salts as the activeindicator.

[0015] A second aspect of the present invention resides in the use, as ahumidity indicator, of a compound comprising a silica-based carriercontaining an iron(II) and/or iron(III) salt or salts as the activeindicator.

[0016] According to a third aspect of the present invention there isprovided a method of monitoring the humidity level within an atmospherecomprising exposing a dried silica-based carrier containing an iron(II)and/or iron(III) salt or salts to said atmosphere and observing colourchanges therein.

[0017] Typically, humidified silica gel is used as the carrier; howeverother forms of silica may be used in the production of the silica-basedcarrier, e.g. silica hydrogel or dry silica gel. The silica-basedmaterial may have any of the physical forms normally available. Inparticular, the form can be irregular granules or approximatelyspherical beads (often called spherical or beaded silica gel).

[0018] The presence of the iron salt imparts a yellow or amber colour tothe dry silica-based carrier. When the indicator is exposed to moisture,it then adsorbs water and the colour is observed to fade until itbecomes almost colourless when the silica-based carrier is almostsaturated with water. This effect has been observed to be a generaleffect for those iron salts which have been examined.

[0019] Once the extent of exposure of the indicator to moisture hasresulted in a change of colour from yellow/amber to almost colourlessthe silica-based carrier may be processed, e.g. by heating, to restoreits colour, and re-used for humidity monitoring.

[0020] The effect referred to above, i.e. colour change from yellow oramber to almost colourless, has been observed in all iron saltsexamined, e.g. simple iron salts such as ferric sulphate, ferricchloride or ferric nitrate and salts having at least two cations ofwhich one is iron(II) or iron(III), examples of which are ammoniumiron(III) sulphate, ammonium iron(II) sulphate and potassium iron(III)sulphate. The effect has been found to be particularly pronounced forthe double sulphates or alums.

[0021] While not wishing to be bound by theory, the effect is thought tobe related to hydrolysis and formation of coloured, polymeric Fe-hydroxyspecies. In dry silica, such species are thought to be polymerised, andalso bound to the silica, to a greater extent than in humidified gel.The higher the degree of polymerisation, and possibly also bonding tothe silica, the more intense the colour.

[0022] The effect would appear to be related to pH. Those iron saltswhich exhibit higher pH values when dissolved in water give more intensecolours, and more pronounced colour changes, than those which exhibitlower (i.e. more acidic) pH values, possibly due to a higher degree ofpolymerisation of the Fe-hydroxy complexes. Thus ammonium iron(III)sulphate at 10% by weight in water has a pH of 1.7 and produces asilica-based carrier with a deep amber colour, whereas a 10% solution offerric chloride has a pH of 1.3 and results in a paler yellow shade. Thecolour of the simple salts can be enhanced by adjusting the pH to highervalues, comparable to the alums. This may be achieved by the addition ofsmall amounts of sodium hydroxide solution.

[0023] Normally an iron(III) salt is employed; however, ferrouscounterparts of the ferric salts may also be used since the ferrous ionreadily oxidises to the ferric state.

[0024] Typically, the silica gel used has a BET surface area in therange of 200 to 1500 m²/g. The pore volume of silica gel may be in therange of 0.2 to 2.0 ml/g, as measured by nitrogen absorption. Forexample, Sorbsil desiccant gel (Sorbsil is a Trade Mark of CrosfieldLimited) typically has a surface area of about 800 m²/g and a porevolume of about 0.4 ml/g. Surface area is determined using standardnitrogen adsorption methods of Brunauer, Emmett and Teller (BET).

[0025] The amount of iron present in the silica-based carrier ispreferably at least about 0.01 percent by weight of iron, determined asFe, relative to the dry weight of the carrier, typically up to about 2.0percent and usually in the range of about 0.01 percent to about 1.0percent by weight of the dry weight of the silica-based carrier. The dryweight of a prepared humidity indicator, based on silica gel, accordingto the invention can be determined by placing a weighed sample (approx.20 grams) in an oven at 145° C. for 16 hours and then weighing the driedmaterial.

[0026] According to another aspect of the present invention there isprovided a method of producing a humidity indicator comprising soakingsilica-based carrier with a solution of an iron(II) and/or iron(III)salt to secure impregnation of the carrier and drying the impregnatedcarrier.

[0027] Typically the indicator gel is prepared by contacting thesilica-based carrier with a solution of iron salt containing 1 percentby weight or higher (up to the saturation point) of the iron salt, e.g.by soaking humidified white silica gel in the iron salt solution.Humidified gel is preferred, but the use of dry gel is acceptable. Whendry gel is used, the granules decrepitate, so that the product has asmaller particle size than the original product, but, generally, theparticle size is still satisfactory for use as a drying agent. In thecase of ammonium iron(III) sulphate (herein referred to as iron(III)alum), the solution may range from 1 percent to approximately 50 percentby weight (saturation at 25° C.), or higher at higher temperatures.Preferably, the solution contains 10 to 40 percent by weight iron(III)alum at 25° C. The use of a high concentration of iron salt helps toreduce the processing time for preparing the indicating silica-basedproduct. The gel is typically soaked in the solution for a period offrom 10 minutes to 10 days, preferably 1 to 30 hours, more preferably 2to 24 hours. The excess solution is drained and the gel dried at 105 to230° C. whereupon it develops its amber colour. An impregnated productdried in this manner will usually have a weight loss after heating at145° C. for 16 hours of less than 10 percent by weight. Preferably, theweight loss at 145° C. is less than 2 percent by weight.

[0028] The invention is illustrated by the following, non-limitingexamples.

EXAMPLE 1

[0029] Sorbsil silica gel (commercially available from Crosfield Limitedof Warrington, England) was exposed to humidity or steam until the poresystem was totally saturated with water transported from the vapourphase. 50 g of this humidified gel was impregnated with a ferric salt bysoaking the gel in 200 ml of 20 percent by weight iron(III) alumsolution for 24 hours. The gel was drained and then dried at 145° C. for16 hours. 6 g samples of the impregnated, dried gel were placed in aseries of glass tubes and air at various levels of relative humidity(RH) passed through the gel for 7 hours at a flow rate of 4litres/minute. After exposure to moisture-containing air for this lengthof time, the colour of the gel samples was measured using a MinoltaCR200 Chromameter, calibrated to a standard white plate and using CIEIlluminant C and a 2° observer angle. The results, expressed accordingto the L*, a*, b* system, are given in Table 1 below. TABLE 1 % RH %weight gain L* a* b* 0 0.0 39.96 10.96 32.38 20 8.4 44.40 8.95 35.47 4014.9 48.13 4.39 23.53 50 18.2 50.00 1.67 15.90 80 25.6 59.94 −0.30 12.30

[0030] The increase in lightness (L*) and decrease in redness (a*) andyellowness (b*) is apparent from the above data and is readily observedvisually, allowing an obvious indication of when the gel has becomesaturated with moisture. Visually, the gel appears almost colourlessafter exposure to 50% RH air at 4 litres/min for 7 hours.

EXAMPLE 2

[0031] A further batch of silica gel was prepared according to themethod of Example 1 and the gel was soaked in the ferric alum solutionfor 4 hours, rather than 24 hours. The product was similarly exposed tomoist air and the results are given in Table 2 below. TABLE 2 % RHColour % weight gain L* a* b* 0 Deep amber 0.0 40.77 +13.23 35.13 20Pale amber 11.7 45.62 +6.94 36.90 40 Yellow 20.6 56.48 +0.59 23.94 50Pale Yellow 25.4 53.64 −0.28 17.90 80 Almost colourless 31.0 57.93 −1.7414.28

[0032] This product shows an improvement over Example 1 especially interms of water adsorption capacity.

EXAMPLE 3

[0033] Samples of ferric salts were placed in the oven at 145° C. for 16hours to observe the effect of dehydration and to see if any colourchanges observed matched those obtained with silica-based carriersimpregnated with the ferric salts. Observations are given below in Table3. TABLE 3 Salt Colour prior to drying Colour after drying Ferric alumpale lilac light buff Ferric sulphate light buff light buff Ferricchloride deep yellow dark brown (decomposed to ferric oxide) Ferricnitrate pale violet dark brown (decomposed to ferric oxide)

[0034] Where colour changes were observed, it was found that they didnot correspond to those seen in the impregnated carrier. This indicatesthat the colour change observed in the carrier impregnated withiron(III) salts is not due to a simple hydration/rehydration effect aswith cobalt and copper salts.

EXAMPLE 4

[0035] Silica gel was impregnated with various iron salts using asimilar method to that described in Example 1. The details of thereaction conditions are given in Table 4 below. In these laboratoryexperiments particular care was taken to remove as much excess solutionas possible from the gel using tissue before the oven drying. Duringoven drying the treated materials were spread in as thin a layer aspossible. This was found to give a product with a more homogenouscolour. TABLE 4 Solution Ratio gel/ Soaking Sample Iron salt strengthsolution time A Potassium iron alum 10%  50 g/200 ml  24 hours B Ferricsulphate 40%  50 g/200 ml  24 hours C Ferric chloride 10% 100 g/200 ml2.5 hours D Ferric nitrate 10% 100 g/200 ml 2.5 hours

[0036] The samples were exposed to moist air as described in Example 1and the results are given in Table 5 below. TABLE 5 Sam- % ple RH %moisture Colour L* a* b* A 0 0 Amber 50.97 +7.54 +35.06 20 10.5 Amber49.55 +6.61 +34.73 40 20.9 Yellow 54.34 +2.00 +25.10 50 23.6 Almostcolourless 59.63 −0.23 +17.80 80 26.1 Almost colourless 60.20 +0.29+18.02 B 0 0 Yellow/amber 41.94 +4.15 +29.37 20 11.0 Yellow 53.22 +2.55+30.21 40 19.4 Yellow 53.75 +1.32 +27.22 50 21.5 Pale yellow 57.67 +1.14+26.74 80 23.8 Almost colourless 55.01 −0.32 +22.54 C 0 0 Amber 39.02+10.12 +32.63 20 11.2 Yellow/amber 48.05 +4.20 +28.30 40 23.5 Paleyellow/amber 52.35 +2.88 +25.76 50 27.1 Pale yellow/amber 54.62 +1.79+23.67 80 31.5 Pale yellow/amber 55.45 +1.46 +23.29 D 0 0 Amber 44.49+7.17 +31.10 20 10.8 Pale amber 45.79 +6.47 +29.90 40 23.2 Pale amber53.71 +4.14 +28.64 50 26.6 Pale amber 51.47 +3.62 +26.16 80 29.6 Paleyellow 53.74 +2.68 +25.34

[0037] The chloride and nitrate show a less marked colour change thanwhen alum is used. Nevertheless some lightening of colour is visible tothe human eye and the trend can still be detected using the L*a*b*system.

Example 5

[0038] 50 g of humidified gel, prepared as in Example 1, was soaked in200 ml of a 20 percent by weight solution of ammonium iron(II) sulphatefor 4 hours and dried as in Example 1. The product was exposed to moistair as in Example 1 and the observed colour changes are shown in Table 6below. TABLE 6 % RH % water absorbed Colour L* a* b* 0 0 Amber 44.09+15.81 +43.11 20 11.4 Amber 44.97 +14.44 +40.84 40 23.4 Pale amber 52.82+8.43 +36.41 50 26.6 Pale yellow/ 52.05 +6.08 +32.99 amber 80 30.0 Paleyellow 57.91 +2.62 +27.56

[0039] Ammonium iron(II) sulphate has the normal green colour of ferroussalts. However, silica gel integrated with it and dried has the ambercolour associated with iron(III) salts.

EXAMPLE 6

[0040] Samples of commercially available beaded silica gel from threesuppliers were humidified and then impregnated with 20% iron(I) alumsolution for 7 hours, dried at 145° C. overnight and the colourrecorded. Samples were then placed in a desiccator at 100% relativehumidity for a week and the colour recorded.

[0041] The desiccant silica gel beads used in this experiment, and theirsuppliers, were: Bead type/size Supplier “TS6”, 2-5 mm QingDao HaiYangChemical Group Co. Ltd., 7 Mian Yang Road, QingDao, China. 2-5 mm SilgelPackaging Ltd., 2 Horton Court, Hortonwood 50, Telford, Shropshire, UK.Ca. 1-3 mm Engelhard Corp., 600 E. McDowell Road, Jackson, MS 39204,U.S.A.

[0042] Colour changes for the desiccant beaded silica gel impregnatedwith iron(III) alum are given in Table 7 below. TABLE 7 Supplier ColourL* a* b* Haiyang before Amber 46.05 +10.10 +37.91 exposure afterexposure Almost colourless 53.10 −1.58 +18.05 Silgel before Amber 48.69+13.64 +43.57 exposure after exposure Almost colourless 61.07 −2.16+16.04 Engelhard before Amber 50.27 +16.18 +51.36 exposure afterexposure Almost colourless 58.03 −1.35 +15.50

[0043] In each case the beaded silica gel shows a pronounced colourchange from amber when dry to almost colourless when humidified. This isthe same behaviour as is observed when an irregular grannular silica gelis used.

EXAMPLE 7

[0044] Dry silica gel, when placed in water (or an aqueous solution), isknown to decrepitate. However, decrepitation is not necessarily aproblem in the preparation of acceptable indicating silica gel. Todemonstrate this, 50 g dry silica gel having a size range ofapproximately 2.5 to 6.0 mm was soaked in 200 ml of 20 percent by weightiron(III) alum solution for 4 hours and then dried at 145° C. overnight.The colour of the resulting gel was measured before and after being leftin a desiccator at 100% relative humidity for 3 weeks. The colourchanges are shown in Table 8 below. A sieve analysis was carried outbefore and after the impregnation step to demonstrate the effect ofdecrepitation on particle size distribution. The results are shown inTable 9 below. TABLE 8 Colour L* a* b* Before exposure Amber 58.32 +9.97+53.29 After exposure Pale yellow 66.67 −2.84 +20.06

[0045] TABLE 9 Particle Weight % before Weight % after size (mm)impregnation impregnation >5.6 4.74 0.00 3.55-5.6  63.31 0.71  1.6-3.5531.91 36.17 1.0-1.6 0.04 36.35 0.5-1.0 0.01 22.40 <0.5 0.00 4.37

[0046] There had been some breakdown in particle size as a result of thedecrepitation but this does not interfere with the humidity indication.The gel in this example still showed the expected amber to nearlycolourless colour change and the particle size distribution was stillacceptable for normal desiccant use.

EXAMPLE 8

[0047] 100 kg of humidified 2.5-6.0 mm silica gel prepared as in Example1 was soaked in 180 litres of 20 percent by weight iron(III) alumsolution for 4 hours. A pump was used to keep the solution circulatingat a rate of between 25 and 50 litres per minute. The gel was thenwithdrawn, allowed to drain and then dried at 150° C. overnight in 2 cmdeep trays in an oven. Colour and adsorption capacity were measured forthe fresh material as in Example 1. Analysis showed it to contain 0.34%Fe.

[0048] 200 g of orange indicator gel, made as described above, wereplaced in a bowl in a desiccator containing water. The relative humidityin this desiccator was nearly 100% at 25° C. After about two weeksexposure to this high humidity the gel had decolourised. It was thenoven dried at 145° C. overnight and the process repeated. This exposureand regeneration was carried out ten times. After the ten cycles ofhumidity and drying the colour and adsorption capacity of the gel weremeasured again as in Example 1 and compared to the original material.The results are shown below in Tables 10 and 11. TABLE 10 Effect oncolour. Sample L* a* b* Fresh before exposure 42.80 +11.28 +37.71 afterexposure 61.92 −1.02 +15.18 After 10 cycles before exposure 41.74 +11.48+37.86 after exposure 56.72 −1.64 +12.58

[0049] TABLE 11 Effect on adsorption capacity. Sample % RH % wateradsorbed Fresh 20 11.6 40 22.9 50 28.0 80 31.2 After 10 cycles 20 10.140 22.5 50 27.6 80 30.8

[0050] Visually, the fresh and regenerated gels were indistinguishable.There had been no deterioration in colour shade, intensity ordistribution and the colour change effect was also uneffected. Inaddition, the adsorption capacities show no adverse change after tenregeneration cycles.

[0051] Similar recycle results can be obtained with other iron salts butit has been found that the drying temperature may need to be kept belowabout 100° C. when certain salts (e.g. FeCl₃) are used, in orser toaboid the development of an uneven colouration.

What is claimed is:
 1. A humidity indicator comprising a silica gel carrier containing an iron(II) and/or iron(III) salt or salts as an active indicator, the iron salt being a salt containing more than one cation of which one is iron(II) or iron(III).
 2. An indicator as claimed in claim 1 in which the iron salt is ammonium iron(III) sulphate, ammonium iron(II) sulphate or potassium iron(III) sulphate.
 3. An indicator as claimed in claim 1 in which the iron(II) or iron(III) salt is present in an amount in the range 0.01 percent to 2.0 percent by weight expressed as Fe based upon weight of dry carrier.
 4. A humidity indicator as claimed in claim 1 in which the silica gel is humidified silica gel.
 5. A humidity indicator as claimed in claim 1 in which the silica gel is beaded or granular silica gel.
 6. A method of monitoring the humidity level within an atmosphere comprising exposing a dried silica gel carrier containing an iron(II) and/or iron(III) salt or salts to said atmosphere and observing the colour changes therein, the iron salt being a salt containing more than one cation of which one is iron(II) or iron(III).
 7. A method as claimed in claim 6 in which the iron salt is ammonium iron(II) sulphate, ammonium iron(II) sulphateee or potassium iron(III) sulphate.
 8. A method of producing a humidity indicator comprising soaking silica gel carrier with a solution of an iron(II) and/or iron(III) salt or salts to impregnate the carrier and drying the impregnated carrier, the iron salt being a salt containing more than one cation of which one is iron(II) or iron(III).
 9. A humidity indicator in the form of an iron impregnated silica gel carrier produced by the method as claimed in claim
 8. 10. A humidity indicator composition comprising a silica gel carrier containing ferric sulphate or ferric nitrate.
 11. A method of monitoring the humidity level within an atmosphere comprising exposing a dried silica gel carrier containing ferric sulphate or ferric nitrate to said atmosphere and observing colour changes therein.
 12. A method of producing a humidity indicator comprising soaking silica gel carrier with a solution of iron sulphate to secure impregnation of the carrier and drying the impregnated carrier.
 13. A method as claimed in claim 12 in which the colour of the indicator is enhanced by pH adjustment to higher values.
 14. A method as claimed in claim 12 including incorporating alkali to adjust pH to higher values.
 15. A method as claimed in claim 13 in which pH adjustment is effected by sodium hydroxide addition.
 16. A composition as claimed in claim 10 in which the iron salt is present in an amount in the range 0.01 percent to 2.0 percent by weight expressed as Fe based upon weight of dry carrier.
 17. A humidity indicator compound comprising a silica based carrier containing a simple iron salt as the active indicator and in which the colour of the indicator is enhanced by pH adjustment to higher values.
 18. A compound as claimed in claim 17 in which the iron salt comprises ferric sulphate.
 19. A compound as claimed in claim 17 in which the carrier comprises silica gel.
 20. A compound as claimed in claim 19 in which the silica gel is humidified silica gel.
 21. A compound as claimed in claim 19 in which the silica gel is beaded or granular silica gel.
 22. A method of monitoring the humidity level within an atmosphere comprising providing a dried silica based carrier which contains a simple iron salt and in which the colour of the indicator is enhanced by pH adjustment to higher values, exposing said carrier to said atmosphere and observing colour changes therein.
 23. A method as claimed in claim 22 in which pH adjustment is effected by sodium hydroxide addition.
 24. A compound as claimed in claim 22 in which the iron salt comprises ferric sulphate. 